A camera typically comprises a photosensor, such as a CCD (charge coupled device) or CMOS (complimentary metal on silicon) photosensor, on which light from a scene imaged by the camera is focused by the camera's optics during an exposure period of the camera to acquire an image of the scene. The photosensor typically comprises an array of rows and columns of light sensitive pixels that register the light focused by the camera optics on the photosensor. Amounts of light registered by the pixels are determined and used to provide the image of the scene.
A pixel in the photosensor registers incident light from a region of the scene imaged on the pixel by the optics by accumulating positive or negative electric charge provided by electron-hole pairs generated in the pixel by the incident light. Charge provided by electrons or holes from electron-hole pairs generated by light is often referred to as “photocharge”. The electron-hole pairs may be generated in a depletion zone of a photodiode comprised in the pixel and the electrons, or holes, are transferred to a storage region of the pixel adjacent the photodiode. Applying a voltage, optionally referred to as a “transfer voltage”, to a conductive “transfer” gate overlying the storage region transfers the electrons or holes from the photodiode to the storage region. Photocharge accumulated in the storage regions of pixels in the photosensor is converted to voltage, and a set of voltages provided by the pixels may be used to produce an image of the scene. The set of voltages provided by the photosensor may be referred to as a “frame” of the photosensor. A doping structure of the semiconductor material comprised in the photosensor determines whether pixels in the photosensor accumulate electrons or holes generated by incident light. Usually pixels accumulate electrons, conventionally also referred to as photoelectrons, originating from electron-hole pairs to register incident light.
The exposure period of a camera is generally controllable so that for given imaging conditions under which a scene is imaged, such as focal length of the camera optics and light available from the scene, pixels in the camera's photosensor register sufficient amounts of light to provide a satisfactory image of the scene. For example, for pixels in the photosensor to register sufficient light to provide a satisfactory image of a dimly lit scene, the camera may advantageously be controlled to acquire light from the scene during a relatively long exposure period. To image a brightly lit scene, a relatively short exposure period may be sufficient.
Some special purpose cameras may operate with special constraints on exposure periods. For example, a time of flight (TOF) three dimensional (3D) range camera acquires range images of scenes that it images. The range images provide distances to features in the scenes. The camera determines a distance to a feature in an imaged scene by determining how long it takes light to make a round trip from the camera to the feature and back to the camera. The round trip times may be determined by transmitting pulses of light to illuminate the scene and determining how long it takes light in the transmitted light pulses reflected by the features to propagate from the camera to the features and back to the camera. The camera may register light from the transmitted light pulses that returns from the scene to the camera during each of a plurality of different exposure periods to acquire data for determining the round trip time. The exposure periods may be required to meet relatively stringent constraints on their respective durations, and their respective timing relative to transmission times of the light pulses.
Durations of camera exposure periods are generally functions of sensitivity of pixels in the camera's photosensor to incident light. A photosensor comprising pixels characterized by greater sensitivity to light is generally operable to acquire satisfactory images of a scene for intensities of light from the scene that are lower than light intensities advantageous for imaging the scene using a photosensor having pixels of lesser light sensitivity. Pixel sensitivity to incident light generally increases as size of the photodiode in the pixel increases. For a given exposure period a pixel having a larger photodiode accumulates more photocharge than a pixel having a smaller photodiode. A photosensor comprising larger photodiodes may therefore be able to provide satisfactory images of a scene at lower light intensities than a camera comprising pixels having smaller photodiodes. However, as photodiodes in a photosensor of a camera increase in size, spatial resolution of the photosensor, and an image it produces, decreases.